Recently, display devices comprising a thin-film transistor (TFT) as a semiconductor device have been put into practical use. As examples of display devices, a liquid crystal display device and an organic electroluminescent display device are considered. These types of display devices are increasingly required to narrow the frame of the display portion year by year.
To fulfill this demand, miniaturization should be realized by improving the driving performance of the semiconductor device. To enhance the driving performance of the semiconductor device, for example, improvement of field-effect mobility of the semiconductor, acquisition of a thin gate insulating film, and reduction of parasitic capacitance can be considered. However, the improvement of mobility will cause the manufacturing throughput to be decreased. When the thickness of the gate insulating film is reduced, yields may be decreased. Thus, in fact, it is difficult to apply these methods.
As a measure to reduce the parasitic capacitance, a semiconductor device comprising an insulated gate field effect transistor is suggested. In this semiconductor device, the number of wiring contact portions for the source region of the transistor is greater than the number of wiring contact portions for the drain region. The interval between the contact portion of the gate line for the gate electrode and the wiring contact portion for the drain region is set to be greater than the interval between the contact portion of the gate line for the gate electrode and the wiring contact portion for the source region.
However, when the above semiconductor device is applied to a pixel transistor or a transistor for color selection used for a liquid crystal display, etc., the voltage relationship of the source and the drain may be reversed (in other words, inversion drive). Thus, it is difficult to obtain an effect of reduction of parasitic capacitance.